Generally, when using infrared energy to measure gas amounts within a sample, the infrared energy passes through the sample in a sample cell and is detected by an infrared sensor. The infrared energy will be reduced by the presence of any gas that absorbs infrared energy. The wavelength band of the beam of infrared energy passing through the sample cell containing the gas sample is changed periodically by the selective interposition of one or more filters in the path of the beam. The filters are usually carried on a rotatable wheel. Typically, each filter passes only radiation at a narrow band corresponding to a characteristic absorption wavelength band of a particular gas of interest. Therefore, through use of an infrared filter that is selected for each gas to be monitored in the gas sample, only infrared energy that can be absorbed by that gas is allowed to pass through the filter and be detected by the sensor.
With this arrangement, ideally the only infrared energy changes, as compared to the infrared energy detected when the gas to be monitored is not present, would be caused by the particular gas to be monitored. However, in practice and as one example, the changes in infrared energy detected may be caused by the infrared source itself changing its power output. The power output of the infrared source can change as the gas analyzer as a whole heats up and as the power source ages. To ensure accuracy in gas analysis, these types of changes in the infrared source power output have to be handled in some particular manner.
Several methods for handling the power output changes in the infrared source are known such as served heaters, extra sensors and stopping the gas analysis frequently to recalibrate the analyzer. These methods are typically complex and costly and have one thing in common, they try to monitor the infrared power output of the infrared source by reading it with some other sensor or some other path not changed by the gases to be monitored. The problem inherent in these methods lies in the fact that a different sensor/path from that of the sensor/path analyzing the gases is used to monitor changes in the power output of the infrared source. Because a different sensor/path is utilized, these methods are not the most accurate.
For example, one particular method of determining infrared source output is to use a second or additional filter to look at a wavelength in the infrared spectrum that is not changed by the gas to be monitored or any other gas that could be in the sample cell when the gas analyzer unit is in normal use. This method however may not produce the best results for the following reasons. As the infrared source ages, the power output spectra changes. Therefore, wavelength where gas absorption is being checked may change in one direction and the wavelength where power level changes may change in the other direction.
Another problem with gas analyzers is the susceptibility of filters to heat. All infrared filters are affected by heat. With changes in temperature, these filters shift in the spectra being passed and change in transmittance so that some of the gas absorption signal they are designed to pass will be lost when the filter shifts.
The use of served heaters minimizes changes within the infrared analyzer, however, significantly increased cost, power consumption and heat buildup makes served heaters not a preferred method for minimizing changes in power output of the infrared source.